ABSTRACT
The sun is known to guide homing pigeons as a priority cue. The literature indicates that under total overcast conditions pigeons rely on a backup mechanism akin to the magnetic inclination compass for which there is much laboratory evidence in migratory birds. Total overcast conditions are not favorable for orientation research in the State of Sao Paulo, Brazil. The orientation of homing pigeons raised near the tropic of Capricorn was therefore observed around the time of the December solstice, when the sun culminates directly overhead, with a consequent interruption of the sun compass for a short time every day. In these experiments, carried out between 1981 and 1993, local geomagnetic field inclination was -25 degrees Celsius to -29 degrees Celsius 30', so that a functioning magnetic inclination compass should have been available to the birds. Whereas the birds released with sun to zenith angles between 10 degrees Celsius and 30 degrees Celsius were well oriented, both in the morning (99 vanishing bearings) and in the afternoon (143 vanishing bearings), those released with the sun less than 5 degrees Celsius away from the zenith showed random orientation (105 vanishing bearings), with no evidence of an alternative magnetic compass mechanism.